The invention relates to a method and a device for increasing the engine brake power of a reciprocating piston internal combustion engine of a vehicle, particularly of a diesel engine, comprising at least one cylinder with at least one inlet valve and one exhaust valve in each case, a turbine, a compressor, an air compressor, at least one storage device, a charge air line and a control device.
With the increasing degree of boosting of diesel engines boosted by turbocharger devices having a turbine and a compressor, the reduction in engine capacity and size are made. Engine brake power, referred to hereinafter as “brake power,” is also being reduced. However, in the case of boosted diesel engines, a brake power which increases adequately with engine power must in all cases be available. A lack of adequate brake power arises in particular, therefore, with the current “downsizing” of engines in which large-capacity, heavy engines are being replaced by small-capacity, lighter engines with significantly increased specific power output.
For this reason the central problem which must be solved when “downsizing” is, above all, that of generating high brake power, which should correspond to that of the larger engine, in order not to overload the conventional brake system while traveling downhill, for example, or impair the usual driving comfort.
At the same time, however, decreases of brake power in normal driving which occur with frequent reductions of engine load and speed, which large engines can partly bridge with their flywheel effect, must be compensated in the case of smaller engines with rapidly available brake forces generated in the cylinder.
So-called exhaust throttle valves, which make possible increased exhaust backpressure and therefore improved engine brake power at high engine speeds by more or less completely closing the exhaust gas line in order to achieve high engine braking moments, are known in the prior art. A disadvantage of this simple technology is that the brake power is achieved predominantly only by the throttling losses of the exhaust gases pushed back and forth in the more or less sealed chamber between piston head and exhaust valve, which process, apart from modest efficiency gains—the maximum achievable brake power equals approximately 50% of engine power—also leads, above all, to undesired heating of the exhaust and injection valves, which are highly stressed thermally in any case. For this reason a substantially improved brake power of up to more than 100% of engine power is achieved by systems which exploit the compression work of the engine by venting the compressed combustion air at the end of the compression stroke by briefly opening the gas exchange valves or by a separate, controlled “brake valve” in the cylinder head, which combustion air can therefore no longer act as a work accumulator which re-exerts on the descending piston (that is, during the working stroke of the ignited engine) the compression work stored in the aspirated combustion air.
It is already clear from the above that the quantity of air introduced into the cylinder during braking operation is a measure for the compression work and therefore also for engine brake power, in these effective engine brake systems.
This effect is reinforced by the fact that during braking operation, also called overrun operation—especially in the case of boosted engines in which no charge pressure is present in this operating state—engines work with relatively poor degrees of cylinder filling, which result from the flow resistances in the intake system and are progressively increased by the elevated engine speeds during braking operation. Moreover, precisely in the case of boosted engines the compression ratio must be significantly lowered as compared to naturally aspirated engines (e=21 to e=16) in order to limit ignition pressures, which also leads to a significant reduction in compression work and therefore in brake power.
It is also known, in vehicles with diesel engines with a compressed air brake system, to draw compressed air from a compressed air storage device which is separated from the brake system proper for safety reasons, the supply of this additional quantity of injection air being generated by an enlarged air compressor, as compared to the standard compressed air brake system, or by boosting with surplus boost air from the engine. This “additional air” is supplied to the engine in the intake system, that is, before or after the turbocharger, to improve acceleration. It is also known that an increase in torque in the low-load range can be achieved by this method. A disadvantage, however, is the high consumption of air which results from the fact that the additional air is not supplied to the individual cylinders in a specified and timed manner. This disadvantage is avoided by the most recent air injection systems known to the applicant, which inject the required quantity of additional air in a timed manner with electronically controlled and regulated pneumatic components, which may be integrated in the engine electronics, for example in the electronically controlled fuel injection system.
It is therefore the object of the invention to improve the engine brake power of a reciprocating piston internal combustion engine of such a vehicle.
The invention provides a method by which additional air is supplied in a timed manner in the braking phase to each cylinder of the engine individually or to the intake tract as a whole.
The systems already developed for increasing engine power and torque are extended to the effect that, in combination with the same or similar mechanical, pneumatic and electronic components for increasing torque, cylinder filling together with engine brake power is increased in a simple manner by means of timed air injection during braking operation, so that the compression work and also, in combination with the known venting devices, the brake power is increased significantly above the state of the art, and the aforementioned disadvantages with boosted and therefore smaller-capacity engines are thus eliminated or at least considerably reduced.
A method according to the invention for increasing the engine brake power of a reciprocating piston internal combustion engine of a vehicle, in particular of a diesel engine, comprising at least one cylinder with at least one inlet valve and one exhaust valve in each case, a turbine, a compressor, an air compressor, at least one storage device, a charge air line and a control device, is characterized by the following procedural steps:
compressing of air from a charge air line or from a second air inlet by the air compressor;
storing of the air compressed by the air compressor in at least one storage device; and
timed injection of injection air, which is stored as compressed air in the at least one storage device and/or is delivered from the air compressor, into the cylinder in order to increase the compression work so as to enhance the engine brake power during a braking process.
It is thereby advantageously achieved that the quantity of additional injection air is consumed only in an order of magnitude which corresponds to the brake power of the engine at the time. A saving in storage space for this injection air and in the associated compressor output is also achieved thereby. This method is suited to vehicles with and without a compressed air brake system.
In the case of vehicles with a compressed air brake system it is especially advantageous that, during the procedural step of storing, the compressed air is first fed to a first storage device and stored therein, and that the air stored in the first storage device is transferred to a second storage device via a feed valve for storage in the second storage device when a given quantity of air at a given pressure is present in the first storage device.
In an embodiment of the present invention the feed valve is controlled by the control device, whereby it is advantageously ensured that the compressed air brake system does not suffer a compressed air loss. At the same time, monitoring of the pressure is possible.
In a preferred embodiment according to the invention, the procedural step of the timed injection comprises the following partial steps:
determining by the control device of an operating state of the engine and the vehicle with reference to data of an engine control computer and/or to suitable sensors;
sensing of a pressure in the at least one storage device by a sensor and/or via a pressure regulator, of a charge pressure in the charge air line and of an engine speed, which correspond to a braking operation, and transmission of this information to the control device;
injecting of injection air by opening a control valve to the inlet valve of the cylinder by the control device for injecting injection air into the cylinder when the inlet valve is opened and an operating state of the engine is present during a braking process;
ending the injection of injection air into the cylinder when the braking process is ended.
In this configuration the particular advantage lies in the timed injection of the additional injection air as a function of brake power actually required. An injection of additional air advantageously takes place only when it is needed. A large saving is thereby achieved.
In a further embodiment a time segment for opening the control valve by the control device is determined, in the partial step of injecting, by a predefinable or stored data value. It is thereby achieved that the injection air is superimposed on the flow of charge air present in the inlet duct and a temperature exchange can therefore take place between these gases, which therefore also advantageously counteracts overheating of the parts close to the combustion chamber. Furthermore, it is advantageously achieved through this predefinable time segment that, for a given duration of injection, the latter is ended early enough, so that no backflow of injection air from the cylinder into the intake system or the charge air line takes place and causes disturbances therein.
In an especially preferred configuration the control device adjusts the quantity of injection air by the pressure regulator as a function of the operating state of the engine and the vehicle at the time. An especially effective increase in engine brake power is thereby achieved, since the injection quantity is dependent on a plurality of operating parameters. To this end it is also a major additional advantage that the quantity of air injected into the engine is adjusted by the control device as a function of the required engine brake power with reference to predefinable stored table values in a mutually coordinated manner.
In a preferred configuration an inlet of the air compressor is connected via a change-over valve to a second air inlet or to the charge air line as a function of a pressure prevailing in the charge air line at the time. The capacity of the air compressor is thereby advantageously increased and use of a larger and more expensive air compressor avoided.
A device for increasing the engine brake power of a reciprocating piston internal combustion engine of a vehicle, in particular of a diesel engine, comprising at least one cylinder with at least one inlet valve and a brake valve in each case, a turbine, a compressor, an air compressor, a storage device, a charge air line and a control device, is characterized in that an outlet of the storage device is connected via a control valve to an inlet duct or to the intake tract of the engine. By means of the control valve it is possible to control the injection air in an advantageously simple manner, since this valve is opened by the control device only when injection of injection air is necessary on the basis of operating conditions.
In a vehicle with a compressed air brake system an inlet of a second storage device is connected via a feed valve to a first storage device. The compressed air brake system with its storage device and its compressed air generating capacity is thereby also usable for the compressed air generation of the injection air, the second storage device representing a particular security measure for the compressed air brake system since it forms a separate compressed air circuit for injecting the compressed air stored therein.
In a preferred configuration the control valve and the outlet of the second storage device are connected via a pressure regulator, said pressure regulator making it possible, via adjustment of the pressure of the injection air which flows through it during injection, to adjust the quantity of injection air in a simple manner.
It is advantageous if the air injection line is connected via an injection duct or an injection line to the inlet duct, the injection duct or injection line being formed in the cylinder head of the engine or arranged in the inlet duct, since specified injection, for example independently of the pressure conditions in the charge air duct, is thereby achieved.
In a further embodiment, a heat exchanger is arranged in the connecting line from the outlet of the second storage device to the injection duct or to the injection line. Via this heat exchanger the injection air can advantageously be cooled during braking operation and thereby contribute to reducing the thermal stress on the engine.
The invention is explained with reference to an exemplary embodiment and to the appended drawing. The single FIGURE shows a schematic representation of parts of an engine with associated components, with an exemplary configuration of the device according to the invention for carrying out the method according to the invention.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawing.